1. Field of Invention
The present invention relates to a cable drop compensation circuit, in particular, relates to a DC output power circuit using the cable drop compensation circuit.
2. Related Prior Art
In general, a cable resistance exists in a power cable between a power supply and a load and, therefore, there's a voltage drop between the power and the load. The voltage drop is proportional to a current of the load. An excessive voltage drop will make a load voltage received by the load lower than an acceptable voltage range for a normal operation, thus causing a problem that the load cannot work properly.
In order to compensate for the voltage drop of the power cable, a conventional power supply compensates for the voltage drop according to the load current so that the load voltage received by the load is kept at a predetermined value of load voltage. Referring to FIG. 1, there is a power conversion circuit with cable drop compensation of which details are disclosed in the U.S. Patent Publication No. US20130027987. The power conversion circuit includes a transformer 10, power transistors 20 and 75, a pulse width modulation (PWM) controller 30, a photo coupler 60, a synchronous rectification controller 70, and a regulation circuit 100, and is used for converting an input voltage Vin into an output voltage Vo. The power transistor 20 is coupled to a primary winding Np of the transformer 10 and the ground, and is used for switching the transformer 10. The PWM controller 30 generates a switching signal Spwm to switch the power transistor 20 to adjust the output voltage Vo according to a feedback signal VFB. An output capacitor 45 is coupled to a secondary winding Ns of the transformer 10 for regulating the output voltage Vo. The photo coupler 60 is coupled to the secondary winding Ns via a resistor 62 and generates the feedback signal VFB to the PWM controller 30 based on the output voltage Vo. The secondary winding Ns of the transformer 10 is coupled to the load to provide an output voltage Vo via a power cable, and a load current Io goes through the power cable.
The power conversion circuit has a synchronous rectification circuit to improve the conversion efficiency thereof. The synchronous rectification circuit includes a synchronous rectification controller 70 and a power transistor 75, and the power transistor 75 has a parasitic diode 76. The power transistor 75 is used to replace traditional diodes for rectification, wherein the drain of the power transistor 75 is coupled to the secondary winding Ns, and the parasitic diode 76 is arranged between the drain and the source of the power transistor 75. The synchronous rectification controller 70 generates and sends a synchronous rectification signal Ssr to the gate of the power transistor 75 to turn on and off the power transistor 75.
The regulation circuit 100 receives the synchronous rectification signals Ssr and a voltage divider signal VA to generate and send a signal VF to the photo coupler 60. The voltage divider signal VA is generated by a voltage divider consisting of resistors 51 and 52 based on the output voltage Vo. Therefore, the feedback signal VFB is generated according to the output voltage Vo. Furthermore, a resistor 115 is coupled to a terminal Rp of the voltage regulation circuit 100. The synchronous rectification signal Ssr is related to the output current Io, and the compensation amount can be programmed by the resister 115. Thus, the output voltage Vo can be adjusted according to the synchronous rectification signal Ssr and the resistor 115 for compensating the voltage drop of the power cable.
However, such circuit structure of the voltage regulation circuit 100 is complicated, and cannot be applied to multiple loads that suffer from different power cable voltage drops.